The present invention relates to a method and apparatus for filling a pulp tower. The invention is especially well applicable in the wood processing industry to filling high consistency pulp towers and corresponding storage towers containing fiber suspension.
Pulp towers used in the wood processing industry are, as know, most commonly tanks containing high-consistency pulp, the consistency being 10-20%, although pulp at a lower consistency is also used occasionally. These tanks are used for example for storing pulp or as blow tanks of some apparatus, i.e. for example for storing pulp which comes in batches from batch digesters, the pulp being then used as a uniform flow in the subsequent treatment apparatus. In other words, it is characteristic of towers according to the invention that the level thereof varies to a great extent, although they most commonly have an optimum level, and the intention is to keep the surface of the pulp at this level.
Several different arrangements for filling pulp towers of the above-mentioned type are known from the prior art. One of the oldest methods known is pumping the pulp to the top of the tower, wherefrom it is allowed to drop down more or less directly. If the pulp is allowed to drop directly onto the pulp below, it goes without saying that the pulp dropping from high above permeates the surface of the pulp in the tower and penetrates deep down into the old pulp. There are several drawbacks to this. For the first, if a dilution of the pulp is performed in the lower portion of the tower, as is very often the case, the pulp fed to the tower may permeate as far as to the dilution zone. This results in the pulp discharging uncontrollably to the dilution zone and the dilution not being as uniform as would be required for the apparatus following the tower. Another problem is that the pulp, when permeating into the old pulp, is drifted closer to the discharge opening of the tower than the pulp already present in the tower, whereby the content of the tower does not change uniformly, but part of the pulp passes out of the tower within some minutes, whereas another part may have to stay in the tower even as long as several weeks. This, in turn, brings about more problems. For the first, it is impossible to even imagine that pulp staying in a tower for days or even weeks could be of the same quality as fresh pulp. For the second, a complete change of stock in towers like this may take days and at least several hours, the pulp discharged from the tower being thus a mixture of old and new pulp of this time span. Depending on the following object of use of the pulp, this "intermediate pulp" may in the worst case be totally unusable. Furthermore, the old pulp staying longer in the tower and the new pulp permeating deeper into the tower, liquid is gradually filtered from the surface of the pulp layer, whereby the surface layer hardens and gets thus damaged more easily. Finally, energy consumption may also be mentioned as a problem, at least from the point of view of our invention, since pumping the whole production volume of a pulp mill, i.e. about 1,000 tons of pulp per day, to the height of 20-30 meters, only to be dropped down to the height of about 3-10 meters, can be regarded as wasting of pumping energy. In other words, the pumping energy that would be really needed is most often less than half of the energy now used.
It is of course possible (U.S. Pat. No. 3,964,962) to discharge the pulp onto a distributing device, for example onto a rotating plate arranged in the upper portion of the tower, by means of which plate the pulp is distributed more uniformly all over the cross-section of the tower. Part of said problems may be solved in this way, but pumping energy is still consumed to the same extent as before, and in addition, the arrangement of distributing devices of pulp in the upper portion of the tower results in both complicated structures and great energy consumption. As the distributing device decomposes the pulp flow into drops, or at least into relatively small particles, a significant amount of air is bound to the pulp during the down-drop, which air will have to be removed at later states of the process by means of vacuum pumps consuming a great deal of energy, or by means of corresponding arrangements.
In connection with some towers, it has been taken into account that the tower is rarely full, and therefore the energy consumption may be decreased by leading the feed pipe of the pulp from the side of the tower into the inside thereof. If the feed pipe is above the pulp surface, the pulp being supplied through the supply opening penetrates through the surface of the pulp layer in the tower into the pulp column at least within some distance, whereby the same problems with pulps of different ages still exist even if they are not as difficult as in the towers filled from the top.
The next alternative is to dispose the feed pipe on the side of the tower below the pulp surface, though directed in such a way that the pulp is supplied substantially onto the pulp layer in the tower. This arrangement works exactly as long as the surface level in the tower remains unchanged. If the surface level varies several meters, the problem may be, for example, that the pulp fed into the tower remains several meters below the pulp surface, whereby the pulp on the surface level cannot discharge, and the new pulp being fed passes first to the discharge. Correspondingly, if the surface level drops a lot below the feed opening, the pulp being fed into the tower penetrates deep into the pulp layer, causing the very problems described above. In other words, this method of feeding is suitable for such cases only where the surface level of the tower remains relatively stable.
Another problem relating to the filling of pulp towers emerges in connection with batch cooking processes, and in particular with the filling of the blow towers thereof. It is generally known that when emptying a batch digester, the consistency of the pulp being discharged from the digester varies to a great extent. Hereby, areas of pulp having different consistencies are formed also in the blow tower, no matter by which prior art method it is filled, and this leads in most cases to variation in the consistency of the pulp directed to the process stage following the tower. This, in turn, causes various problems in brown stock washing, for example. As known, for example washers are dimensioned for an optimum consistency and deviating from this inevitably weakens the operation result.
Various problems caused by the prior art arrangements described above can be solved by means of the method and apparatus of our invention, the characteristic features of which become apparent from the appended claims.